Implement theme mode utility functions

To simplify the usage of styled-theming's `theme()` (1) and
`theme.variants()` API functions, two utility functions has been
implemented:

- `themeMode(modeStyles : object)` — Shorthand function for the `theme`
  API to define styles based on the global theme mode(s).
- `themedModeVariant(modeStyles : object, variantPropName : string)` —
  Shorthand function for the `theme.variants` API to define variant
  styles based on the global theme mode(s).

References:
  (1) https://github.com/styled-components/styled-theming#themename-values

GH-53

src/styles/theme/index.js

@@ -16,16 +16,21 @@
 
 import globals from "./globals";
 import normalize from "./normalize";
+import { themedMode, themedModeVariant } from "./utils";
 import typography from "./typography";
 import { MODE_BRIGHT_SNOW_FLURRY, MODE_DARK_NIGHT_FROST, THEME_KEY_MODE } from "./constants";
 
-const theme = {
+const theme = { typography };
+
+export {
+  globals,
+  normalize,
+  themedMode,
+  themedModeVariant,
   typography,
   MODE_BRIGHT_SNOW_FLURRY,
   MODE_DARK_NIGHT_FROST,
   THEME_KEY_MODE
 };
 
-export { globals, normalize, typography };
-
 export default theme;

src/styles/theme/utils/index.js

@@ -0,0 +1,20 @@
+/*
+ * Copyright (C) 2018-present Arctic Ice Studio <[email protected]>
+ * Copyright (C) 2018-present Sven Greb <[email protected]>
+ *
+ * Project:    Nord Docs
+ * Repository: https://github.com/arcticicestudio/nord-docs
+ * License:    MIT
+ */
+
+/**
+ * @file Provides theme utilities.
+ * @author Arctic Ice Studio <[email protected]>
+ * @author Sven Greb <[email protected]>
+ * @since 0.2.0
+ */
+
+import themedMode from "./themedMode";
+import themedModeVariant from "./themedModeVariant";
+
+export { themedMode, themedModeVariant };

src/styles/theme/utils/themedMode.js

@@ -0,0 +1,31 @@
+/*
+ * Copyright (C) 2018-present Arctic Ice Studio <[email protected]>
+ * Copyright (C) 2018-present Sven Greb <[email protected]>
+ *
+ * Project:    Nord Docs
+ * Repository: https://github.com/arcticicestudio/nord-docs
+ * License:    MIT
+ */
+
+/**
+ * @file Provides a shorthand function to define styles based on the global theme mode(s).
+ * @author Arctic Ice Studio <[email protected]>
+ * @author Sven Greb <[email protected]>
+ * @since 0.2.0
+ */
+
+import styledTheming from "styled-theming";
+
+import { THEME_KEY_MODE } from "../constants";
+
+/**
+ * Shorthand function to define styles based on the global theme mode(s).
+ *
+ * @method themedMode
+ * @param {object} modeStyles The global theme mode style(s).
+ * @return {function} The styles as interpolated function for usage in "styled-components" scope.
+ * @see https://github.com/styled-components/styled-theming#themename-values
+ */
+const themedMode = modeStyles => styledTheming(THEME_KEY_MODE, modeStyles);
+
+export default themedMode;

src/styles/theme/utils/themedModeVariant.js

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2018-present Arctic Ice Studio <[email protected]>
+ * Copyright (C) 2018-present Sven Greb <[email protected]>
+ *
+ * Project:    Nord Docs
+ * Repository: https://github.com/arcticicestudio/nord-docs
+ * License:    MIT
+ */
+
+/**
+ * @file Provides a shorthand function to define variant styles based on the global theme mode(s).
+ * @author Arctic Ice Studio <[email protected]>
+ * @author Sven Greb <[email protected]>
+ * @since 0.2.0
+ */
+
+import styledTheming from "styled-theming";
+
+import { THEME_KEY_MODE } from "../constants";
+
+/**
+ * Shorthand function to define variant styles based on the global theme mode(s).
+ *
+ * @method themedModeVariant
+ * @param  {object} modeStyles The global theme mode style(s) for each variant.
+ * @param  {string} variantPropName The name of the prop which allows to define variants. Defaults to `variant`.
+ * @return {function} The styles as interpolated function for usage in styled-components scope.
+ * @see https://github.com/styled-components/styled-theming#themevariantsname-prop-themes
+ */
+const themedModeVariant = (modeStyles, variantPropName = "variant") =>
+  styledTheming.variants(THEME_KEY_MODE, variantPropName, modeStyles);
+
+export default themedModeVariant;